| In this study,the atrazine-degrading strain Arthrobacter sp.DNS10 was selected as the tested strain.The effects of three different nitrogen fertilizers addition?nitrate nitrogen-sodium nitrate,ammonium nitrogen-ammonium sulfate,and amide nitrogen-urea?on DNS10 degrading atrazine residues in maize farmland black soil was investigated.The main results are as follows:?1?By measuring the amount of atrazine residues in each treatment soil sample,it was found that: On the 3rd day of the remediation stage,the residual amount of soil atrazine in the remediation treatment group with strain DNS10 but no nitrogen fertilizer was 7.75 mg·kg-1,while the residual amount of atrazine in the soil samples with the addition of strains DNS10 and sodium nitrate,ammonium sulfate or urea were 8.95 mg·kg-1,5.03 mg·kg-1 and 4.70 mg·kg-1 respectively.Adding nitrogen fertilizer can improve the efficiency of strain DNS10 degradation of atrazine to some extent,and urea has the best effect on promoting the removal of atrazine residues in the soil by the strain DNS10.Comparing the content of atrazine in maize seedlings in different treatment,the addition of nitrogen fertilizer can significantly reduce the accumulation of atrazine in maize seedlings.Compared with the non-nitrogen remediation treatment group,the content of atrazine in leaves and roots of maize seedlings decreased by 20.5% and 2.1%,respectively in sodium nitrate treatment,decreased by 41.8% and 14.8% in ammonium sulfate treatment and decreased by 42.4%and 36.3% in urea treatment.Scanning electron microscopy showed that the addition of nitrogen fertilizer could promote the formation of biofilm at the root surface of maize seedlings.The biofilm of the roots of maize seedlings treated with urea was more dense,while the biofilms of roots of maize seedlings treated with sodium nitrate and ammonium sulfate were sparse.The results of q PCR analysis show that the expression levels of atrazine chlorohydrolase gene?trz N?in the roots of maize seedlings treated with the above three nitrogen fertilizers were 1.24 times,1.42 times and 1.84 times higher than that of the control treatment?no nitrogen fertilizer?,indicating that the addition of nitrogen fertilizer can promote the expression of trz N in the root surface of maize seedlings.The addition of nitrogen fertilizer can promote the formation of biofilm on the root surface of maize by atrazine-degrading microorganisms and degradation gene expression,promote the reduction of soil atrazine residues and alleviating the transmission of atrazine to maize seedlings.The promotion capacity of urea is better than the other two nitrogen fertilizers.?2?For the growth index of maize seedlings,plant height,dry weight and fresh weight were all inhibited by atrazine,which decreased by 2.1%,8.0% and 5.0% respectively.The addition of strain DNS10 for remediation can alleviate the stress of atrazine on the growth of maize seedlings,so that all indicators can be restored to the level without stress.The addition of sodium nitrate,ammonium sulfate and urea can promote the growth of maize seedlings in the remediation test.The plant height,dry weight and fresh weight were increased by 9.1%-10.4%,67.3%-70.3%,17.4%-44.2% respectively.The three nitrogen fertilizers were similar in effect and all reached significant differences.For the physiological indexes of maize seedlings,the contents of chlorophyll a,chlorophyll b,chlorophyll?a+b?and carotenoids in maize seedlings decreased by 28.2%,48.0%,35.8% and 16.8%,respectively.While the strain DNS10 was added for remediation,it could be restored to the level of unstressed and improved,which increased by 51.6%,37.3%,47.3%,and27.2%,respectively.The four indicators of nitrogen fertilizer treatment increased further,the chlorophyll a content increased by 111.3%-192.1%,the chlorophyll b increased by 164.0%-248.7%,the chlorophyll?a+b?content increased by 129.6%-203.4%,the carotenoid content increased by40.9%62.8%,and all reached a significant difference level compared to the remediation treatment without nitrogen fertilizer.The addition of urea is more effective than the other two nitrogen fertilizers.In the remediation treatment of the added strain DNS10,the MDA accumulation in the roots of corn seedlings with treatment with no nitrogen fertilizer,sodium nitrate,ammonium sulfate and urea was decreased by 23.8%,16.6%,36.0% and 51.4%,respectively.It was proved that the strain DNS10 can alleviate the oxidative stress caused by atrazine on maize seedlings,and the addition of ammonium sulfate and urea can promote this alleviation.The soluble sugar content and total nitrogen content in leaves of maize seedlings were less affected by atrazine,but were more affected by microorganisms.The soluble sugar content and total nitrogen content,which were not contaminated by atrazine and added DNS10,increased by 8.3% and 31.7%,respectively.The addition of nitrogen fertilizer promoted the microbial activities of maize seedling roots and provided sufficient nutrients,which further increased the soluble sugar content and total nitrogen content of maize seedlings,and the three nitrogen fertilizers promoted similar effects.The addition of atrazine reduced the total phosphorus content in the leaves of maize seedlings and the total phosphorus content of the leaves was not significantly improved after adding strain DNS10.The addition of ammonium sulfate during remediation can restore the leaf content of maize seedlings to a level that is not affected by atrazine stress.Comprehensively investigate the effects of different nitrogen fertilizers on the degradation of atrazine and the effects of the degradationr process on the physiological state of maize seedlings,it can be concluded that the addition of urea was the best in relieving the stress of atrazine on maize seedlings during the degradation of atrazine by strain DNS10.?3?Four kinds of EPS?EPS-A,EPS-B,EPS-C,EPS-D?produced by cultured strain DNS10 in the basic salt medium,sodium nitrate salt medium,ammonium sulfatesalt medium and urea salt medium by heating extraction method.After the addition of different EPS?EPS-A,EPS-B,EPS-C,EPS-D?to the soil,the adsorption amount of atrazine in the soil increased to different degrees,which was 14.6%,33.65%,51.7% and 69.5% higher than that without EPS.The adsorption process can be well fitted with the kinetic quasi-secondary model and the Freundlich isotherm model.Chemical adsorption and chemical bonding are the main effects of the absorption of atrazine on black soil.The addition of sodium nitrate,ammonium sulfate and urea promoted the increase of protein content in EPS,which was 2.33 times,3.06 times and 3.13 times higher than CK treatment,respectively.The results of three-dimensional fluorescence spectrometry also showed that the fluorescence intensity of proteinaceous substances in EPS produced by strain DNS10 after three nitrogen additions increased significantly.The fluorescence intensity of proteinaceous substances was significantly quenched by the combination of different EPS and atrazine,indicating that protein plays an important role in the binding of EPS to atrazine,and the addition of sodium nitrate,ammonium sulfate and urea will cause strain DNS10 to produce more proteinaceous substances that can bind to atrazine.The promoting effect is expressed as urea > ammonium sulfate > sodium nitrate.At the same time,the results of infrared spectroscopy showed that the new absorption peak caused by the C=O bond stretching vibration in the protein appeared at 1650 cm-1 after the three nitrogen additions,It indicated that the addition of sodium nitrate,ammonium sulfate and urea could promote the secretion of extracellular polymer with C=O bond structure by strain DNS10,and the result could consistent with the increase of protein content in EPS.After the four EPSs were combined with atrazine,the vibration peaks drifted at 1800-1500 cm-1,1385±10 cm-1,and1150-1000 cm-1.That is,when EPS absorbs atrazine,functional groups such as hydroxyl,carboxyl and carbonyl groups in proteins and polysaccharides play a role.The adsorption process of EPS and atrazine is inseparable from protein and polysaccharide,and protein plays a stronger role than polysaccharide. |
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